(1) Field of the Invention
The present invention generally relates to a solid state image sensing device, and more particularly to a solid state image sensing device in which photo-electric charge generated in photo-diodes is stored therein and information corresponding to the photo-electric charge is read out therefrom by electronic scanning.
In recent years, to increase the number of pixels in a solid state image sensing device and to arrange the pixels with a high density, it is required that all the pixels have uniform characteristics. Thus, improvements in the production process of the image sensing device have been attempted. On the other hand, it is difficult to obtain uniform characteristics of photo-diodes which are made, for example, of compound semiconductor. In this case, output signals from the solid state image sensing device are corrected by complex processing in an external circuit.
In the solid state image sensing device field, it is desired that variation of output signals depending on dispersion of characteristics of the photo-diodes is reduced without complex processing in an external circuit.
(2) Description of the Related Art
In a conventional solid state image sensing device having photo-diodes, a single bias voltage is supplied to the respective photo-diodes and electric charge corresponding to the quantity of light to each photo-diode and affected by the dark current of each photo-diode is stored therein and read out therefrom. Thus, in an external circuit coupled to the solid state image sensing device, dark current characteristics of the respective photo-diodes have been previously stored in a semiconductor memory, and the output signal from each pixel is corrected based on a dark current characteristic stored in the semiconductor memory for a corresponding photo-diode.
As has been described above, when the conventional solid state image sensing device is used, the dispersion of the dark currents of the respective photo-diodes must be compensated for by the external circuit. Thus, the external circuit coupled to the solid state image sensing device is complex and a long processing time is needed in the external circuit.
In addition, if the bias voltage is set to a value suitable for a photo-diode having a small dark current, the storage capacity of electric charge in a photo-diode having a large dark current is insufficient so that an output signal from a corresponding pixel is saturated. In this case, this pixel is a defective pixel. On the other hand, if the bias voltage to be supplied to the photo-diodes is set to a low value to prevent pixels corresponding to photo-diodes having a large dark current from being defective, the sensitivity of the solid state image sensing device deteriorates.